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A. The usual suspects in the growth of pond scum are not common algae, of which there are many kinds, but a kind of microorganism once called blue-green algae and now usually referred to as cyanobacteria, because they are more like bacteria in structure. Cyanobacteria are normally unobtrusive year-round residents of puddles, ponds and lakes and usually do not make themselves known until they undergo a spurt of abnormal population growth called a bloom, producing visible scum.

Enormous blooms have clogged lakes and waterways from Florida to China. Normally, though, small blooms die off after a week or so. The factors leading to a bloom are not fully understood, but are believed to include at least intermittent exposure to bright sunlight and warm temperatures; calm, cloudy water; and an enriched supply of nutrients, notably phosphorus and nitrogen.

Summer brings increased sunlight, which powers the photosynthesis the cyanobacteria use to make their own food. When light and temperature drop off in the fall, so does the growth of cyanobacteria. No one factor predicts a bloom, but an excess of nutrients from sources like runoff from fertilized agricultural land and sewage is assumed to be a major contributor and is a likely target for human control efforts. Control of large-scale blooms is a priority, because some products of cyanobacteria, released when the cells die, are potentially poisonous to people and animals.